Delimbing machines are used to strip the branches off (i.e. "delimb") felled trees and to cut the delimbed tree into logs of selected length for subsequent transportation to log processing plants. Delimbing machines conventionally incorporate a slidably positionable boom which is rotatably and pivotably mounted above a processing platform. A pair of grapple arms are mounted at the free end of the boom. The machine operator extends the boom and manipulates it to position the open grapple arms around a felled tree. After closing the grapple arms tightly around the tree, the machine operator raises the boom to lift the grappled tree off the ground and then slidably retracts the boom. This action propels the tree along its longitudinal axis relative to a cutting mechanism which delimbs the tree. The boom and tree are then slidably extended and the grapple arms opened to free the delimbed tree from the machine. Optionally, the operator may actuate a cutting mechanism to cut the delimbed tree into selected lengths at one or more intervals as the boom and delimbed tree are extended.
The art has evolved two basic types of delimbing machines. "Stroke delimbers" are configured and operate essentially as described above; one or more supporting arms being provided on the processing platform to support the free end of the tree at points displaced from the grapple arms as the tree is longitudinally propelled with the aid of the sliding boom. In an effort to increase productivity, a second type of tree delimbing machine known as a "roll-stroke delimber" has been developed. Roll-stroke delimbers incorporate a mechanism for continuously feeding the free end of the tree (i.e. the end which is displaced from the grapple arms) thereby reducing stresses imparted to the machine and to the machine operator by conventional stroke delimbers and significantly increasing productivity, particularly in the processing of long trees.
Continuous tree feeding mechanisms conventionally comprise a pair of feed rollers which are pressed against opposite sides of the tree and counter-rotated relative to one another, thereby propelling the tree along its longitudinal axis toward or away from the delimbing machine, depending upon the direction of rotation of the feed rollers. In order to ensure adequate traction between the tree and the feed rollers, various gripping mechanisms have been provided on the feed roller outer surfaces. For example, prior art delimbers have employed log feed rollers studded with spikes which bite into the tree outer surface as the rollers are rotated against the tree. Unfortunately, the spikes penetrate not only the tree bark but also the wood of the tree, damaging the wood and reducing production yields.
Rubber covered log feed rollers have also been employed in prior art delimbers in an effort to eliminate the wood damage caused by spiked rollers. Although rubber rollers facilitate damage-free log feeding, production problems are experienced if the rubber rollers are unable to attain adequate traction against the surface of the tree. For example, if the tree bark is removed, or if the tree is wet, then the rollers tend to slip against the surface of the tree, without adequately feeding the tree relative to the delimbing machine, thus diminishing production yields.
As a compromise, the art has evolved spiked chains which are sometimes used instead of feed rollers. The spiked chains are driven around extended tracks arranged on opposite sides of the tree. Because the area of each chain which contacts the tree is significantly greater than the contact area of the aforementioned rollers, comparatively short spikes can be provided on the chains to obtain tractive force approximating that of spiked rollers. The shorter chain spikes cause considerably less wood damage than spiked rollers, but there is nevertheless some wood damage which is preferably avoided altogether.
Yet another prior art continuous tree feeding mechanism utilizes a pair of ribbed, contoured feed rollers. Such rollers are of approximately hourglass- shaped cross section, as distinct from the generally straight-edged cross section of the cylindrical spiked or rubber feed rollers discussed above. The space between the opposed, hourglass-shaped rollers defines a guideway which varies in cross section as the tree is fed through the rollers. That is, the shape of the guideway is more or less continuously conformed to the shape of the tree at the point between the rollers, thereby maximizing the roller-to-tree contact area throughout the delimbing operation. The roller outer surfaces are provided with extended knife-like ribs in order to obtain sufficient tractive force to propel the tree back and forth relative to the cutting mechanism which delimbs the tree. Unfortunately, some wood damage unavoidably results with this arrangement as well.
Those skilled in the art will appreciate that there is a need for a log feed roller mechanism capable of increasing the throughput of a delimbing machine while minimizing wood damage.